1. Field of the Invention
The invention relates to a plastic-packaged semiconductor integrated circuit, and more particularly to a TSOP (thin small outline package) type semiconductor integrated circuit having a LOC (lead on chip) structure. The invention also relates to a method of fabricating such a plastic-packaged semiconductor integrated circuit.
2. Description of the Related Art
An example of plastic-packaged semiconductor integrated circuits having a LOC structure is illustrated in FIG. 1A. Inner leads 5 extend inwardly of a package over a pellet on which electronic circuits (not illustrated) are mounted. The inner leads 5 extend from outside of the package toward the pellet 1 past an outer edge of the pellet and reach a region where the electronic circuits are fabricated. The pellet 1 and the inner leads 5 are stuck to each other through electrically insulating adhesive tapes 2. The inner leads 5 are bonded to bonding pads 3 formed at a surface of the pellet 1 through wires 4. The inner leads 5, the pellet 1 and the wires 4 are all shielded with an outer packaging resin layer 20. A plurality of outer leads 17 formed continually with the inner leads 5 extend outside the package. The outer leads 17 act as an external terminal for electrically connecting the pellet 1 to an external circuit. As illustrated in FIG. 1A, the outer leads 17 are bent downwardly of the package along the resin layer 20, and further bent outwardly of the package. In other words, the outer leads 17 are shaped gull-wing.
In a conventional plastic-packaged LSI, inner leads are disposed surrounding a pellet, and thus tip ends of the inner leads are located relatively remote away from the pellet. Accordingly, the tip ends of the inner leads are remote away from bonding pads formed at a surface of the pellet, resulting in that it was difficult to make an entire package smaller iii size. However, the semiconductor integrated circuit illustrated in FIG. 1A makes it possible to make an entire package smaller in size, because tip ends of the inner leads 5 extend above the pellet 1.
Besides the gull-wing outer leads 17 as illustrated in FIG. 1A, some LOC-structured TSOP type LSIs have J-shaped outer leads as illustrated in FIG. 1B. It is common to the semiconductor integrated circuits illustrated in FIGS. 1A and 1B that the inner leads horizontally extend from the pellet outwardly of the package, form external terminals as the outer leads, and are bent along the resin layer to thereby form the gull-wing or J-shaped leads.
The above mentioned LOC-structured TSOP type LSIs have a problem that it is difficult to make an entire package smaller in size, and that they are not suitable for accomplishing a higher chip-mount density with which they are mounted onto a substrate such as a print circuit board. Such a problem is posed due to a geometric reason that outer leads acting as external terminals have to be extended outwardly of a resin layer, and further due to a structural reason that a certain distance is required between an outer edge of the resin layer and the pellet for supporting the outer leads in order to give bent-resistance to the outer leads.
Apart from the above mentioned LOC-structured TSOP LSI, there is known a plastic-packaged LSI in which external terminals are designed to have a ball grid array (BGA) structure to thereby attempt to make an entire package smaller in size. In BGA type LSI, an epoxy substrate on which a pellet is to be mounted is formed at a surface thereof with a plurality of solder balls below the pellet, and shielded with resin so that the solder balls are partially exposed outside. The solder balls are directly, electrically connected to pads formed at a surface of the pellet, and the solder balls are located below a lower surface of the pellet, resulting in that it is no longer necessary for external terminals to extend from inside of a resin layer to outside of a package unlike the above mentioned LOC-structured TSOP type LSI. Thus, a BGA structure is suitable for making a package smaller in size.
However, a conventional BGA type LSI poses a problem that, when an epoxy substrate on which a pellet has been already mounted is shielded with resin by means of upper and lower molds, it is impossible to apply a high pressure required for using thermosetting resin. Hence, thermoplastic resin has to be used for shielding a pellet. However, thermoplastic resin is less moisture proof than thermosetting resin, resulting in that it is not avoidable that BGA type LSI are less moisture proof than LOC-structured TSOP type LSI.
In LOC-structured TSOP type LSI, since inner and outer leads extend from inside to outside of a resin layer, moisture contained in a package can escape outside of a package through an interface between the resin layer and the inner and outer leads. On the other hand, BGA type LSI has no path through which moisture contained in a package can escape outside of a package. In this standpoint, it is not avoidable for BGA type LSI to be less moisture proof.